Replace hook_help() hacks with a cleaner solution for the dynamic headers on issue views

Currently, we use an evil, ugly hack in hook_help() inside project_issue to add the dynamic headers on the issue queue views. This is evil and wrong for many reasons. It also leads to silly bugs like #396114: accessing /project/user gives page not found but shows project table (just no issues). I spoke to merlinofchaos about this at DCDC, and we came up with the following alternative plan:

- extend the attachment display plugin into a "Project issue header" subclass
- in this kind of attachment display, have the options I need to select what kinds of links you want
- in the display plugin's render() method, invoke the right function to generate the desired links
- fix the default issue views to use a properly configured attachment
- rip out the nasty inside project_issue_help()

I haven't messed with attachments yet, but this should hopefully be fairly straight forward. I'll work on this soonish...